1. Field of the Invention
The invention relates to a squelch arrangement for receivers comprising an AM demodulator and an FM demodulator for amplitude-modulation or frequency-modulation operation of the receiver respectively, this squelch arrangement comprising a noise level measuring circuit connected to the output of the FM demodulator for supplying a noise signal when the receiver operates in the amplitude-modulation mode or in the frequency-modulation mode, this noise level signal being applied to a comparison circuit which activates an interrupter circuit for cutting-off the audio-frequency signal recovered by the receiver when the noise level exceeds a certain threshold.
2. Description of the Prior Art
It is known that in the absence of a signal at their inputs, radio receivers produce at their outputs a noise which is very annoying especially for monitors continuously tuned to a certain carrier frequency. The squelch arrangements have for their object to mute the audio-frequency output signal of the receiver when no modulation signal is received and to restore it when a modulation signal appears again.
Some types of squelch arrangements used in frequency-modulation receivers are based on measuring the noise level at the output of the frequency demodulator of the receiver, in a frequency band located outside the frequency band used for the modulation signal. The known squelch arrangement, considered above for a receiver capable of operating in the frequency-modulation or the amplitude-modulation mode, is based on the measurement of the noise level at the output of the frequency demodulator of the receiver, whether the receiver operates in the frequency-modulation or the amplitude-modulation mode. But it has been found that when the receiver receives an amplitude-modulated signal there is certain sensitivity of the squelch arrangement to the modulation depth, which causes an increase in the noise produced by the frequency demodulator when the modulation depth exceeds a certain value resulting in an unwanted muting of the audio signal.